1. Field of the Invention
The present invention relates to an electron gun for a cathode ray tube, which has a grid electrode having apertures of improved shapes to form an improved main lens of the electron gun, thereby increasing the degree of freedom in controlling the shapes of the beam spots focused on the phosphor screen, and improving the shape of the electric field formed in the main lens to thereby improve various focusing characteristics of the electron gun. The present invention further relates to an electron gun for a cathode ray tube, which can compensate for the astigmatism caused by the enlarged focusing lens and can advantageously control the static convergence.
2. Prior Arts
In general, an electron gun of a cathode ray tube is an apparatus for radiating three electron beams through a mask so as to form a predetermined image on a phosphor screen. FIG. 1 is a schematic constructional view of the conventional in-line type electron gun for describing its construction and operation. Referring to FIG. 1, in the conventional in-line type electron gun, thermions radiated from a cathode 1 are attracted to a control grid G1 by the voltage applied to a screen grid G2, with forming electron beams. Then, the electron beams pass through a prefocus lens 2 formed between the screen grid G2 and a focusing grid G3 and then proceed into a main lens 3 formed between the focusing grid G3 and an accelerating grid G4. The electron beams focused by the main lens 3 pass through a shadow mask 4 and then form beam spots on a phosphor screen 5 of a panel faceplate 6.
In the meantime, a large-sized color cathode ray tube is increasingly demanded and is trending towards a flat shape. Therefore, it requires a high anode voltage in order to obtain focused spots on the screen with constantly maintaining the speed of the electron beams from the cathode. A high voltage difference between this high anode voltage and the focusing voltage forms an electrostatic lens with a shorter focal distance, which is capable of causing an aberration. Further, when the electron beams pass through the deflection yoke to be deflected to the peripheral portions of the screen by the deflection magnetic field, the distribution of the deflection magnetic field makes the electron beams be under-focused in the horizontal direction and over-focused in the vertical direction.
In order to compensate for the deterioration of the resolution at the peripheral portions of the screen by the above described effects, the conventional electron gun has rim electrodes opposed to each other to form the main lens. Moreover, in the conventional electron gun, the electrode plates having apertures are disposed in positions retreated from the rim electrodes, and the apertures respectively have a shape of a vertically elongated ellipse.
FIG. 2 shows a schematic perspective view of a conventional in-line type electron gun a part of which is cut away. In the conventional in-line type electron gun shown in FIG. 2, the main lens is formed between the G5-2 and the G6 electrodes which contain plate members 22 and 24 respectively having apertures of vertically elongated elliptic shapes.
Also, in the conventional in-line type electron gun as constructed above, the shapes of the beam spots focused on the phosphor screen are elongated in the horizontal direction, which deteriorates the resolution at the peripheral portions of the screen. Although the conventional electron gun also has apertures of vertically elongated elliptic shapes as shown in FIG. 3 in order to eliminate this problem, the resolution at the peripheral portions of the screen does not get improved yet.
In the meantime, the diameter of the main lens of the electron gun generally has a large effect on the focusing characteristic of the color cathode ray tube. In other words, when the diameter of the main lens is small, the spherical aberration of the main lens not only lowers down the resolution at the peripheral portions of the screen but also deteriorates various characteristic in relation to the resolution. Therefore, the larger the diameter of the main lens is, the higher the resolution of the screen is.
However, there is a limit to the size of the neck of the color cathode ray tube in which the electron gun is installed, and therefore, there is a limit to the size of the apertures for forming the main lens. Further, the above inherent structural limit to the size of the apertures becomes more severe nowadays because the diameter of the neck of the color cathode ray tube has a tendency to decrease in order to obtain various advantages such as reduction of the manufacturing cost of the color cathode ray tube, and reduction of the exhaustion of electric power by employing a small-sized deflection yoke.
Several prior arts including U.S. Pat. Nos. 4,583,024, 4,766,344, 4,833,364, and 5,414,323 disclose apertures of various shapes such as a non-symmetric ellipse, a partly cut-away ellipse, a rectangle and a key hole, as shown in FIG. 4, in order to overcome the deterioration of the resolution at the peripheral portions of the screen as described above. The apertures of the proposed shapes achieve the above object of overcoming the deterioration of the resolution to a certain degree. However, the above patents do not pay attention to the influence of the side walls of the grid electrodes on the main lens. Moreover, the above patents do not improve entire focusing characteristics including the degree of freedom in controlling the shape of beam spots in the horizontal and the vertical directions by controlling the shapes of the apertures such as a key hole.
Furthermore, although enlarged main focusing lens has a reduced spherical aberration by providing an easy slope of voltage and although vertical and horizontal beams can reduce the astigmatism, it is difficult to install the electrodes for the main focusing lens at positions retreated a predetermined distance from the rim electrodes.
Moreover, when the electrodes for the main focusing lens are disposed at positions retreated a predetermined distance from the rim electrodes, the horizontal beams make distorted spots because there happens a difference between focusing forces in the directions of the central beam and the side beams.